public function saveInvoice($contens, $preview = true)
 {
     $file = $preview ? ilERP::getPreviewFile() : ilERP::getSaveDirectory() . $this->getInvoiceNumber() . ".pdf";
     $fp = @fopen($file, 'w+');
     if (!$fp) {
         throw new ilERPException("Cannot write " . $file);
     }
     fwrite($fp, $contens);
     fclose($fp);
 }